26S optics. 



line B d for the course of the reflected ray D B. 

 All these reflected rays will meet in a point d, 

 where they will form the extremity d, of the in- 

 verted image e d, similar to the extremity D of the 

 upright object D E. 



If the pencils of rays E f,'E g, E h, be also 

 continued to the mirror, and their angles of re- 

 flection from it be made equal to their angles of 

 incidence upon it, as in the former pencil from D, 

 they will meet at the point e by reflection, and 

 form the extremity e of the image e d, similar to 

 the extremity E of the object D E. 



As each intermediate point of the object between 

 D and E, sends out a pencil of rays in like manner 

 to every part of the mirror, the rays of each pencil 

 will be reflected back from it, and meet in all the 

 intermediate points between the extremities e and 

 d of the image; and so the whole image will be 

 formed not at i f half the distance of the mirror 

 from its centre of concavity C; but at a greater 

 distance between i and the object D E; and the 

 image will be inverted with respect to the object. 



This being well understood, the reader will 

 easily see how the image is formed by the large 

 concave mirror of the reflecting telescope, when 

 he comes to the description of that instrument. 



When the object is more remote from the mirror 

 than its centre of concavity C, the image will be 

 less than the object, and between the object and 

 the mirror; when the object is nearer than the 

 centre of concavity, the image will be more remote, 

 and larger than the object; thus, if D E be the 

 object, e d will be its image; for as the object re- 

 cedes from the mirror, the image approaches 

 nearer to it; and as the object approaches nearer 

 to the mirror, the image recedes farther from it; 



11 



